1. Field of the Invention
The present invention relates to a viewfinder for a single lens reflex camera and, in particular, to a viewfinder having a display for indicating photographing information, or the like, within the field of view of the finder.
2. Description of the Prior Art
In a single lens reflex camera, an image of an object photographed by a photographing lens is exposed to a silver-halide film or is picked up by an image pickup device such as a CCD or a CMOS. The object image is reflected by a main mirror and is converged onto a focusing screen. The object image formed on the focusing screen is enlarged by the finder and can be viewed by a photographer. The object image formed on the focusing screen is inverted into an erect image by a pentagonal prism or pentagonal mirror (image-erecting optical member), so that the photographer can view an enlarged erect image by an eyepiece. In a conventional viewfinder, information, such as a diaphragm value or a shutter speed, is indicated together with the object image in the field of view. The information image which indicates the information is formed by an indicator made of an LCD (Liquid Crystal Display) or another optical element, the information image displayed in the indication device is optically connected to the pentagonal mirror/prism. The connected indication is displayed in the field of view of the finder.
In such a known viewfinder, in order to indicate the information image, an indicator 2 is provided in rear of a mirror box 1, as can be seen in FIG. 7, which shows the internal structure of a camera. An indication prism 9A which is substantially in the form of a triangular column is held by a holder 62 between the indicating surface of the indicator 2 and an incident surface of a pentagonal mirror 7, so that the information image displayed on the indicating surface is transmitted to the inside of the pentagonal mirror 7 through the indication prism 9A. Namely, the indication prism 9A has an incident surface defined by one of the side surfaces forming a triangular column that extends in the vertical direction and faces the incident surface of the indicator 2. One of the remaining surfaces defines an emitting surface which faces the incident surface of the pentagonal mirror 7, and the remaining surface located between the incident surface and the emitting surface defines an inclined reflecting surface. With this arrangement, the information image indicated in the indicator 2 is incident upon the incident surface into the indication prism 9A, is internally reflected by the reflecting surface and is emitted from the emitting surface toward the pentagonal mirror 7. The object image formed by the light bundle transmitted through the photographing lens (not shown) and reflected by the main mirror 5 onto the focusing screen 6 is also incident upon the pentagonal mirror 7. The information image and the object image are inverted into erect images by the pentagonal mirror 7, so that the erect images can be viewed through an ocular optical system 8 by a photographer. In the ocular optical system 8, as the information image and the object image are converged at optically equivalent optical axes (eye points), the photographer can view both the object image and the information image at the same time.
As shown in FIG. 6A, a field frame 101 in which the object image is indicated and a laterally elongated information frame 102 located below the field frame 101 are defined by a field frame plate 10 in the field of view of the finder. The information image indicated in the indicator 2 is indicated in the information frame 102. Note that the field frame 101 is arranged with the lower edge being located away from the indication prism 9A, so that the field frame 101 receives no light transmitted through the indication prism 9A.
As shown in FIG. 7, in a known camera, the indicator 2 has an indication surface extending in the vertical direction (when it is assumed that the optical axis of the photographing lens extends in the horizontal direction) and inclined at an angle θi2 with respect to the incident surface of the pentagonal mirror 7. The angle θi2 is 90° in FIG. 7. Therefore, in the ocular optical system 8, if the information image is made incident upon the incident surface of the pentagonal mirror 7 at a desired incident angle to thereby converge the information image and the object image onto the same image forming position on the optical axis, the optical axis of the indication prism 9A must bend the optical axis of the information image by an angle θt2. Accordingly, the angle of the reflecting surface of the indication prism 9A must be set to θt2/2 with respect to the emitting surface. Since, the deflection angle θt2 is approximately 90°, the angle θ2 defined between the reflecting surface and emitting surface, of the indication prism 9A is approximately 45°. Consequently, the length of the emitting surface of the indication prism 9A in the horizontal direction is increased, thus resulting in an increase in the length of the incident surface of the pentagonal mirror 7 extending toward the center portion (in the left direction in FIG. 7). As can be seen in FIG. 6A, since the field frame 101 in the field of view must be arranged in a position so as not to interfere with the indication prism 9A, the length L2 of the field frame 101 in the height direction within the field of view is limited, whereby the finder coverage (ratio between the field surface area of the object image and the entire field of the finder) is reduced.
As shown in FIG. 7, a shutter 3, provided behind the indicator 2, is provided, on the front upper surface portion which faces the indicator 2, with a recess 34A having a rectangular shape in cross section so that the shutter 3 does not interfere with the indicator 2. Likewise, the pentagonal mirror 7 provided in front of the indicator 2 is provided, on the rear and lower surface portion thereof, with a recess 73A having a rectangular shape in cross section. Hence, the shutter 3 is thin at the portion at which the recess 34A is formed and the pentagonal mirror 7 is thin at the portion at which the recess 73A is formed. Consequently, the mechanical strength of the shutter 3 and the pentagonal mirror 7 is reduced, thus resulting in a reduced reliability thereof. In particular, since the pentagonal mirror 7 is made of a resin mold, the reduction in thickness due to the presence of the recess 73A tends to cause the pentagonal mirror 7 to deform. Such deformation causes distortion of the object image in the field of view. Moreover, in order to provide the shutter 3 without having the recess 34A, if the shutter 3 is displaced rearward, the size of the camera in the forward/rearward direction is increased. In particular, in a digital single lens reflex camera, since a low-pass filter must be provided behind the shutter 3, the camera size is considerably increased to an unacceptable extent with respect to miniaturization of the camera.
To eliminate the drawback of reduction in the field of view, Japanese Patent Publication No. 2000-221568 has proposed providing a transparent element behind the pentagonal prism so that the information image of the indicator is incident upon the pentagonal prism via the transparent element from the indication prism, whereby no light is emitted from the emitting surface of the indication prism toward the incident surface of the pentagonal prism and thus, the field frame can be made as large as possible to prevent the reduction of the filed of view.
The technology in the above-referenced disclosure is useful for a camera having a pentagonal prism. However, in a camera having a pentagonal mirror, it is difficult to guide the information image from the transparent element to the pentagonal mirror, and therefore, it is very difficult to apply such technology to a camera having a pentagonal mirror. Moreover, the technology disclosed in the above-referenced disclosure is useful to increase the finder coverage, but it is difficult to eliminate reduction in the strength of the shutter or the pentagonal mirror, and difficult to achieve miniaturization of the camera. Furthermore, the transparent element, provided as a separate component behind the pentagonal mirror, increases the number of the components and the number of the assembling procedures. Since the transparent element is provided in the ocular optical system, a design modification of the ocular optical system is necessary.